The nuclear body is a multiprotein complex located within the nuclei of cells. The nuclear body is also known as nuclear domain 10, PML oncogenic domain, and the Kr body. Immunohistochemical staining typically indicates 5–30 nuclear bodies within a nucleus. The nuclear bodies appear as discrete, punctate regions. The number of nuclear bodies in the cell, and the intensity of antibody staining of these structures, increase in response to heat shock and viral infection, as well as following exposure to interferons and heavy metals (Ascoli et al., J. Cell Biol. 112:785–795, 1991). The nuclear body appears to be involved in the regulation of gene transcription. Nascent RNA polymerase II transcripts have been found within the nuclear body (LaMorte et al., Proc. Natl. Acad. Sci. USA 95:4991–4996, 1998), and the nuclear body is a preferred site for transcription of viral genes (Ishov et al., J. Cell. Biol. 138:5–16, 1997).
Promyelocytic leukemia (PML) protein is a component of the nuclear body. PML protein is involved in several cellular processes. For example, PML protein regulates cell growth (Wang, et al., Science 279:1547–1551, 1998) and may mediate apoptosis (Wang, et al., Nature Genetics 20:266–272, 1998; Quignon et al., Nature Genetics 10:259–265, 1998). PML protein also recruits cAMP response element-binding protein (the CREB-binding protein or CBP) to the nuclear body and functions as a potent nuclear hormone receptor co-activator (Doucas et al., Proc. Natl. Acad. Sci. USA 96:2627–2632, 1999).
In addition to its involvement in gene transcription, the nuclear body is a target of autoantibodies in the sera of patients who have primary biliary cirrhosis (P BC), an autoimmune disease (Hodges et al., Am. J. Hum. Genet. 63:297–304, 199&; Melnick et al., Blood 93: 3167–3215, 1999; Stemsdorf et al., Immunobiology 198:307–331, 1997). PBC patients carry autoantibodies directed against Sp100 (Speckled 100 kDa), a polypeptide component of the nuclear body (Szostecki et al., J. Immunol. 145:4338–4347, 1990). Two splice variants of Sp100, designated Sp100b and Sp100-HMG, have also been found (Dent et al., Blood 88:1423–1436, 1996; Seeler et al. Proc. Natl. Acad. Sci. USA. 95:7316–7321, 1998; Lehining et al., Proc. Natl. Acad. Sci. USA 95:7322–7326, 1998). These proteins interact with members of the heterochromatin protein 1 (HP1) family of non-histone chromosomal proteins. When bound to a promoter, the Sp100 proteins and HP1 behave as transcriptional repressors in transfected cells. These observations suggest that the nuclear body in general, and the Sp100 proteins in particular, may maintain chromatin architecture and regulate gene transcription (Seeler, et al. Proc. Natl. Acad. Sci. USA 95:7316–7321, 1998 and Lehming et al., Proc. Natl. Acad. Sci. USA 95:7322–7326).
Sera from PBC patients have also been used to identify a leukocyte-specific component of the nuclear body designated Sp140 (Bloch et al., J. Biol. Chem. 46:29198–29204, 1996). The N-terminal portion of Sp140 exhibits sequence homology with the N-terminal segments of the Sp100 proteins. The middle region of Sp140 contains a “SAND” domain (Gibson et al., Trends Biochem. Sci. 23:242–244, 1998), and the C-terminal portion of Sp140 contains a plant homeobox domain and a bromodomain.